1. Field of the Invention
The present invention relates to a granulated powder for use in the powder metallurgy production of a high-density sintered body, a method for producing a high-density sintered body using the granulated powder, and the high-density sintered body produced.
2. Prior Art
Exhaust pipes of automobiles are generally provided with a catalyst device, a mounting boss for an oxygen sensor and the like, which are arranged thereon. Such an exhaust pipe is comprised of, for example, a front pipe, a mounting boss for an oxygen sensor, a catalyst device, and a center pipe which are connected in this order as seen from the exhaust manifold, and the rear end of the center pipe is connected with the muffler. In this arrangement, the front and center pipes are connected with the mounting boss, the catalyst device and the like through flanges that are respectively welded to the front and rear ends of the front and center pipes. Conventionally, these flanges and the mounting boss have been produced by processing ingot materials. Since they have complicated shapes, the processing cost increases, bringing the cost up.
For this reason, these component parts have been produced at lower cost by sintering powder comprised of the same material (e.g., ferrite stainless steel) as that used for the exhaust pipe. That is, a sintered body is produced by a method in which a green compact obtained by pressing a predetermined metal powder charged into a die is sintered at a temperature equal to or higher than the sintering temperature of the metal powder.
With the recent progress of high performance and high efficient engines, the exhaust gas temperature of an engine tends to become higher. Therefore, austenite stainless steel (SUS304, etc.), having excellent heat resistance as compared to that of ferrite stainless steel and having corrosion resistance to the corrosive exhaust gas, is used as a constituent material for the exhaust pipe. When the exhaust pipe comprised of austenite stainless steel is welded to the flange comprised of ferrite stainless steel, however, thermal fatigue properties of the resultant welded article are lowered due to the difference in coefficient of thermal expansion between the constituent materials. This indicates that the flanges and the mounting bosses must be produced by sintering powder of the same material (austenite stainless steel) as that for the exhaust pipe to be welded thereto.
However, the following problems arise when the powders of ferrite stainless steel and austenite stainless steel are sintered.
First, the sintered density (hermetic property) of the resultant sintered body is not high enough. Since component parts of the exhaust system of automobiles require a high hermetic property, such a problem is inconvenient and must be solved. To improve the hermetic property of the sintered body, pores (open pores), in particular, which are present in the sintered body and open to an outer surface thereof, must be reduced. Although the open porosity of the sintered body of stainless steel, constituting the exhaust system parts, must be less than 5% no sintered body having such an excellent hermetic property has been obtained.
The just-mentioned problem becomes remarkable especially when the powder of austenite stainless steel or ferrite stainless steel such as 13Cr-2Al or SUS430J1L is sintered. As for the austenite stainless steel, the diffusion coefficient of Fe contained in the austenite is smaller than that of Fe contained in the ferrite, and the sintering reaction is difficult to proceed, so that the hermetic property of the resultant sintered body is likely to lower. In the case of the ferrite stainless steel powder containing the above-mentioned constituents, the sintering reaction is also difficult to proceed.
Secondly, the green strength of a green compact obtained by compressing the powder prior to the sintering is not adequate, and a crack or breakage is likely to occur, resulting in a tendency that the yield of the final products is lowered. It has been generally known that the powder which is low in flow rate (fluidity) does not uniformly flow in the die during the pressuring, so that the uniformity of the green density (packing density of the powder) is lowered, leading to a low strength of the green compact.
As a countermeasure against the second problem, Japanese Unexamined Patent Publication Nos. 63-293102 and 2-166201, for example, disclose a technique of charging a mixed powder of coarse and fine particles into a die to allow the fine particles to be disposed in spaces between the coarse particles, thus improving the green density. Further, Japanese Unexamined Patent Publication No. 7-90301 discloses a technique of producing granulated powder having an excellent flow rate by combining a main component with a subsidiary component, to thereby improve the green density.
However, as explained above, the stainless steel powder, especially the powder of austenite stainless steel or ferrite stainless steel such as 13Cr-2Al or SUS430J1L, has a tendency that the sintering reaction is difficult to proceed. Therefore, even when the green density during the pressing of powder is increased by employing the conventional technique, the sintered density (hermetic property) of the sintered body cannot be satisfactorily improved.
It is an object of the present invention to solve the above-described problems during the sintering of stainless steel powder, and to provide a granulated powder for a high-density sintered body, which powder has an excellent flow rate (fluidity) in the die during the production of a green compact, so that the resultant green compact has an excellent green strength, making it possible to improve the sintered density (hermetic property) of the sintered body.
It is another object of the present invention to provide a method for producing a high-density sintered body using the above-mentioned granulated powder, and a high-density sintered body produced by this method.
To attain the object, the present invention provides
granulated powder for a high-density sintered body, which powder is obtained by granulation using a binder and a powder that includes small diameter particles comprised of stainless steel and having an average particle diameter of 1 to 20 xcexcm and large diameter particles comprised of stainless steel and having an average particle diameter of 30 to 150 xcexcm,
wherein the content of the small diameter particles in the granulated powder is 30 to 70% by weight, and at least part of the small diameter particles adheres to the surfaces of the large diameter particles.
Preferably, the binder has a lubricating property, and the granulated powder has a flow rate of 15 sec/50 g or less as measured by the determination of flow rate (the fluidity test) employing an orifice diameter of 5 mm in accordance with JIS Z2504.
Further, the present invention provides:
a method for producing a high-density sintered body, comprising the steps of:
producing a green compact using the aforementioned granulated powder; and
sintering the green compact at a temperature of 1100 to 1350xc2x0 C.
Further, the present invention provides:
a high-density sintered body which is a sintered body of the above-mentioned granulated powder and has an open porosity of 5% or less.